This invention relates to a surge protection system for reducing, during power-up, inrush current and overshoot voltage in a shunt-connected filter capacitor at the output of a rectifier which rectifies a-c power line voltage.
With the development of high power solid state devices, large power converters employing rectifier-filter arrangements have become quite common. More particularly, with the improvements made in pulse width modulated inverters and switching power supply techniques, uncontrolled rectifier bridges feeding low pass LC filters are commonly used in large A-C to D-C to A-C power converters. There are two problems, however, when an uncontrolled rectifier bridge, followed by an LC filter, are initially connected to an a-c power distribution system to receive a-c line voltage. The first problem during this power-up period is that the peak current drawn from the a-c power system for the first several line cycles may be large enough to cause fuses and/or circuit breakers in the a-c power system to open. The second problem encountered is the overshoot in the d-c voltage across the shunt-connected filter capacitor. Both of these problems result from the transient response characteristic of the LC filter circuit. A reasonable approximation for the magnitudes of the peak of the current inrush and the amount of the voltage overshoot can be obtained by assuming that the dormant LC filter network is excited by a step of voltage equal to the peak of the a-c voltage on the power distribution system.
Unless the inrush current and the overshoot voltage in the filter capacitor are reduced or limited, in addition to the actuation of the fuses and/or circuit breakers in the a-c power distribution system, the rectifying devices in the rectifier bridge, the filter capacitor itself, and circuit components in the load which is connected to the filter network could be damaged or destroyed. Surge protection systems have been developed in the past to limit the inrush current and overshoot voltage.
In one such prior system, a limiting resistor is inserted in series with each of the power lines, three resistors thereby being required in a three-phase a-c power system. After power-up, a multi-pole contactor is closed to bypass each of the resistors for normal operation, thereby avoiding an undesired power loss and voltage drop. A major disadvantage of this prior system is the number of resistors required to accomplish the limiting. Moreover, the contacts of the shunting contactor must be rated to carry full rated a-c line current.
In another prior system a single limiting resistor, bypassed by a shunting switch, is located on the d-c side of a rectifier bridge. After power-up, the switch is closed and must therefore be sized to conduct the full d-c load current. This is a demanding application for a switch or contactor since the normal d-c load current in a large power converter can be very high.
The surge protection system of the present invention is a substantial improvement over those previously developed. Not only is it of relatively simple construction, but it is significantly less expensive than the prior surge protection systems.